Tree

==Parts and function==

===Roots===
[[File:WisconsinScenery.jpg|thumb|A young [[red pine]] (''Pinus resinosa'') with spread of roots visible, as a result of soil erosion]]
{{main|Root}}

The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree. They are also used for reproduction, defence, survival, energy storage and many other purposes. The [[radicle]] or embryonic root is the first part of a [[seedling]] to emerge from the seed during the process of [[germination]]. This develops into a [[taproot]] which goes straight downwards. Within a few weeks [[lateral root]]s branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the taproot eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell [[root hair]]s. These are in immediate contact with the soil particles and can absorb water and nutrients such as [[potassium]] in solution. The roots require oxygen to [[Respiration (physiology)|respire]] and only a few species such as [[mangrove]]s and the [[Taxodium ascendens|pond cypress]] (''Taxodium ascendens'') can live in permanently waterlogged soil.<ref>{{cite book |title=The World Encyclopedia of Trees |last=Russell |first=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/14 14–15] |url=https://archive.org/details/isbn_9780754812920/page/14}}</ref>

In the soil, the roots encounter the [[hyphae]] of fungi. Many of these are known as [[mycorrhiza]] and form a [[Mutualism (biology)|mutualistic]] relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as [[phosphorus]] from the fungus, while the fungus obtains the [[carbohydrate]] products of photosynthesis from the tree.<ref>{{cite web |url=http://www.waldwissen.net/wald/baeume_waldpflanzen/oekologie/wsl_mykorrhiza_lebensgemeinschaft/index_EN |title=Mycorrhiza&nbsp;– a fascinating symbiosis in the forest |last1=Egli |first1=S. |author2=Brunner, I. |year=2011 |work=Forestknowledge |publisher=Swiss Federal Research Institute |access-date=15 July 2012 |archive-url=https://web.archive.org/web/20130509045021/https://www.waldwissen.net/wald/baeume_waldpflanzen/oekologie/wsl_mykorrhiza_lebensgemeinschaft/index_EN |archive-date=9 May 2013}}</ref> The hyphae of the fungus can link different trees and a network is formed, transferring nutrients and signals from one place to another.<ref>{{Cite journal |last=Heijden |first=Marcel G. A. van der |date=15 April 2016 |title=Underground networking |journal=Science |volume=352 |issue=6283 |pages=290–291 |doi=10.1126/science.aaf4694 |pmid=27081054|bibcode=2016Sci...352..290H |s2cid=133399719 }}</ref> The fungus promotes growth of the roots and helps protect the trees against predators and pathogens. It can also limit damage done to a tree by pollution as the fungus accumulate [[Heavy metal (chemistry)|heavy metals]] within its tissues.<ref>{{cite web |url=https://treesforlife.org.uk/into-the-forest/habitats-and-ecology/ecology/mycorrhizas/ |title=Mycorrhizas |last=Puplett |first=Dan |work=Trees for Life |access-date=15 July 2012 |archive-date=2 November 2019 |archive-url=https://web.archive.org/web/20191102063436/http://treesforlife.org.uk/into-the-forest/habitats-and-ecology/ecology/mycorrhizas/ |url-status=live }}</ref> Fossil evidence shows that roots have been associated with mycorrhizal fungi since the early [[Paleozoic]], four hundred million years ago, when the first [[vascular plant]]s colonised dry land.<ref>{{cite journal |last=Brundrett |first=Mark C. |year=2002 |title=Coevolution of roots and mycorrhizas of land plants |journal=New Phytologist |volume=154 |issue=2 |pages=275–304 |doi=10.1046/j.1469-8137.2002.00397.x|pmid=33873429 |doi-access=free }}</ref>

[[File:Bombax LalBagh.JPG|thumb|left|Buttress roots of the kapok tree (''[[Ceiba pentandra]]'')]]
Some trees such as [[Alder]] (''Alnus'' species) have a [[Symbiosis|symbiotic]] relationship with ''[[Frankia]]'' species, a filamentous bacterium that can fix nitrogen from the air, converting it into [[ammonia]]. They have [[Actinorhizal plant|actinorhizal]] root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive.<ref>{{cite web |url=http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm |title=Frankia and Actinorhizal Plants |last=Benson |first=David |publisher=University of Connecticut |access-date=15 July 2012 |archive-date=6 August 2018 |archive-url=https://web.archive.org/web/20180806064343/http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm |url-status=dead }}</ref> The plant hormones called [[cytokinin]]s initiate root nodule formation, in a process closely related to mycorrhizal association.<ref>{{cite book |title=Signaling in Plants |last=Baluška |first=František |author2=Mancuso, Stefano |year=2009 |publisher=Springer |isbn=978-3-540-89227-4 |pages=83–84 |url=https://books.google.com/books?id=RTFMl0cArgAC&pg=PA83 |access-date=29 May 2020 |archive-date=19 August 2020 |archive-url=https://web.archive.org/web/20200819211216/https://books.google.com/books?id=RTFMl0cArgAC&pg=PA83 |url-status=live }}</ref>

It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the [[inosculation]] process, a kind of natural [[grafting]] or welding of vegetal tissues. The tests to demonstrate this networking are performed by injecting chemicals, sometimes [[Radioactive decay|radioactive]], into a tree, and then checking for its presence in neighbouring trees.<ref>{{cite journal |last=Hough |first=Walter A. |title=Root extension of Individual trees in surface soils of a Natural Longleaf Pine-Turkey Oak Stand |journal=Forest Science |date=1 June 1965 |volume=11 |issue=2 |pages=223–242 |url=http://www.ingentaconnect.com/content/saf/fs/1965/00000011/00000002/art00020 |archive-url=https://web.archive.org/web/20131209062328/http://www.ingentaconnect.com/content/saf/fs/1965/00000011/00000002/art00020 |archive-date=9 December 2013 |url-status=dead }}</ref>

The roots are, generally, an underground part of the tree, but some tree species have evolved roots that are [[aerial roots|aerial]]. The common purposes for aerial roots may be of two kinds, to contribute to the mechanical stability of the tree, and to obtain oxygen from air. An instance of mechanical stability enhancement is the [[Rhizophora mangle|red mangrove]] that develops [[buttress roots|prop roots]] that loop out of the trunk and branches and descend vertically into the mud.<ref name=Singapore>{{cite web |url=http://mangrove.nus.edu.sg/guidebooks/text/1043.htm |title=How plants cope in the mangroves |editor=Ng, Peter K. L. |editor2=Sivasothi, N. |year=2001 |work=Mangroves of Singapore |access-date=15 July 2012 |archive-date=22 May 2012 |archive-url=https://web.archive.org/web/20120522185105/http://mangrove.nus.edu.sg/guidebooks/text/1043.htm |url-status=live }}</ref> A similar structure is developed by the [[Ficus benghalensis|Indian banyan]].<ref name=Thomas>{{cite book |title=Trees: Their Natural History |last=Thomas |first=Peter |year=2000 |publisher=Cambridge University Press |isbn=978-0-521-45963-1 |page=108 |url=https://books.google.com/books?id=w6bUF8bOfKEC&pg=PA108 |access-date=29 May 2020 |archive-date=20 August 2020 |archive-url=https://web.archive.org/web/20200820005841/https://books.google.com/books?id=w6bUF8bOfKEC&pg=PA108 |url-status=live }}</ref> Many large trees have [[buttress root]]s which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface.<ref>{{cite journal |last1=Crook |first1=M. J. |author2=Ennos, A. R. |author3=Banks, J. R. |year=1997 |title=The function of buttress roots: a comparative study of the anchorage systems of buttressed (''Aglaia'' and ''Nephelium ramboutan'' species) and non-buttressed (''Mallotus wrayi'') tropical trees |journal=Journal of Experimental Botany |volume=48 |issue=9 |pages=1703–1716 |doi=10.1093/jxb/48.9.1703|doi-access=free }}</ref>

Some tree species have developed root extensions that pop out of soil, in order to get oxygen, when it is not available in the soil because of excess water. These root extensions are called [[pneumatophores]], and are present, among others, in [[Avicennia germinans|black mangrove]] and pond cypress.<ref name=Singapore/>

===Trunk===
[[File:Buk1.JPG|thumb|upright|Northern beech (''[[Fagus sylvatica]]'') trunk in autumn]]
{{main|Trunk (botany)}}

The main purpose of the trunk is to raise the leaves above the ground, enabling the tree to overtop other plants and outcompete them for light.<ref name="King1990">{{cite journal |last1=King |first1=David A. |title=The Adaptive Significance of Tree Height |journal=The American Naturalist |volume=135 |issue=6 |year=1990 |pages=809–828 |doi=10.1086/285075 |s2cid=85160969 |quote=competition for light is the primary factor responsible for the evolution and maintenance of the arboreal life form. The resulting evolutionarily stable growth pattern maximizes the competitive ability of the individual }}</ref> It also transports water and nutrients from the roots to the aerial parts of the tree, and distributes the food produced by the leaves to all other parts, including the roots.<ref name=Russelltrunk>{{cite book |title=The World Encyclopedia of Trees |last1=Russell |first1=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/16 16–17] |url=https://archive.org/details/isbn_9780754812920/page/16 }}</ref>

In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the [[bark (botany)|bark]], mostly composed of dead cells of [[phellem]] (cork).<ref name="Junikka1994">{{cite journal |last1=Junikka |first1=Leo |title=Survey of English Macroscopic Bark Terminology |journal=IAWA Journal |volume=15 |issue=1 |year=1994 |pages=3–45 |doi=10.1163/22941932-90001338 |quote=phellem{{!}}a secondarily formed protective tissue in stems and roots consisting of dead cells with chiefly suberised<!--made cork-like--> walls: developed outward from the phellogen and forming a part of periderm|doi-access=free }}</ref> It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called [[lenticel]]s, through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the [[cork cambium]] or phellogen.<ref name="Junikka1994"/> The [[Platanus × acerifolia|London plane]] (''Platanus × acerifolia'') periodically sheds its bark in large flakes. Similarly, the bark of the [[Betula pendula|silver birch]] (''Betula pendula'') peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. In some trees such as the [[pine]] (''Pinus'' species) the bark exudes sticky [[resin]] which deters attackers whereas in [[Hevea brasiliensis|rubber trees]] (''Hevea brasiliensis'') it is a milky [[latex]] that oozes out. The [[Cinchona officinalis|quinine bark tree]] (''Cinchona officinalis'') contains bitter substances to make the bark unpalatable.<ref name=Russelltrunk/> Large tree-like plants with lignified trunks in the [[Pteridophyta]], [[Arecales]], [[Cycadophyta]] and [[Poales]] such as the tree ferns, palms, cycads and bamboos have different structures and outer coverings.<ref>{{cite book |title=Biology |last=Campbell |first=Neil A. |author2=Reece, Jane B. |year=2002 |publisher=Pearson Education |isbn=978-0-201-75054-6 |page=725|edition=6th }}</ref>

[[File:Taxus wood.jpg|thumb|left|A section of [[Taxus baccata|yew]] (''Taxus baccata'') showing 27 annual growth rings, pale [[Sapwood (wood)|sapwood]] and dark [[heartwood]]]]

Although the bark functions as a protective barrier, it is itself attacked by boring insects such as beetles. These lay their eggs in crevices and the larvae chew their way through the cellulose tissues leaving a gallery of tunnels. This may allow fungal spores to gain admittance and attack the tree. [[Dutch elm disease]] is caused by a fungus (''[[Ophiostoma]]'' species) carried from one [[elm]] tree to another by various beetles. The tree reacts to the growth of the fungus by blocking off the xylem tissue carrying sap upwards and the branch above, and eventually the whole tree, is deprived of nourishment and dies. In Britain in the 1990s, 25 million elm trees were killed by this disease.<ref>{{cite web |url=http://www.forestry.gov.uk/fr/HCOU-4U4JCL |title=Dutch elm disease in Britain |author=Webber, Joan |publisher=Forest Research |access-date=16 July 2012 |archive-date=9 March 2018 |archive-url=https://web.archive.org/web/20180309165010/https://www.forestry.gov.uk/fr/hcou-4u4jcl |url-status=dead }}</ref>

The innermost layer of bark is known as the [[phloem]] and this is involved in the transport of the [[Plant sap|sap]] containing the sugars made by photosynthesis to other parts of the tree. It is a soft spongy layer of living cells, some of which are arranged end to end to form tubes. These are supported by [[parenchyma]] cells which provide padding and include fibres for strengthening the tissue.<ref name=Lalonde>{{cite journal |author1=Lalonde, S. |author2=Wipf, D. |author3=Frommer, W. B. |year=2004 |title=Transport mechanisms for organic forms of carbon and nitrogen between source and sink |journal=Annual Review of Plant Biology |volume=55 |pages=341–372 |pmid=15377224 |doi=10.1146/annurev.arplant.55.031903.141758}}</ref> Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. The cells are continually dividing, creating phloem cells on the outside and wood cells known as [[xylem]] on the inside.<ref>{{cite web |url=http://cronodon.com/BioTech/Plant_Bodies_Wood.html |title=Wood, tree trunks and branches |work=BioTech |publisher=Cronodon Museum |access-date=16 July 2012 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502084356/http://cronodon.com/BioTech/Plant_Bodies_Wood.html |url-status=live }}</ref>

The newly created xylem is the [[sapwood (wood)|sapwood]]. It is composed of water-conducting cells and associated cells which are often living, and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The oldest, inner part of the sapwood is progressively converted into [[heartwood]] as new sapwood is formed at the cambium. The conductive cells of the heartwood are blocked in some species. Heartwood is usually darker in colour than the sapwood. It is the dense central core of the trunk giving it rigidity. Three quarters of the dry mass of the xylem is [[cellulose]], a [[polysaccharide]], and most of the remainder is lignin, a complex [[polymer]]. A transverse section through a tree trunk or a horizontal core will show concentric circles of lighter or darker wood – tree rings.<ref name=woodanatomy/> These rings are the [[Growth rings|annual growth rings]]<ref>{{cite book |last=Fritts |first=H. C. |title=Tree Rings and Climate |publisher=Blackburn Press |year= 2001 |isbn=978-1-930665-39-2}}</ref><ref>{{cite web |last1=Helama |first1=Samuel |last2=Jalkanen |first2=Risto |title=Annual growth rings of trees |url=https://www.luke.fi/en/natural-resources/forest/forests-and-climate-change/annual-growth-rings-of-trees/ |publisher=Natural Resources Institute Finland (LUKE) |access-date=17 July 2019 |archive-date=6 August 2019 |archive-url=https://web.archive.org/web/20190806043449/https://www.luke.fi/en/natural-resources/forest/forests-and-climate-change/annual-growth-rings-of-trees/ |url-status=live }}</ref> There may also be rays running at right angles to growth rings. These are [[Medullary ray (botany)|vascular rays]] which are thin sheets of living tissue permeating the wood.<ref name=woodanatomy>{{cite web |url=http://www.botany.uwc.ac.za/ecotree/trunk/woodanatomy2.htm |title=The anatomy of a tree trunk – the wood 2 |url-status=bot: unknown |archive-url=https://web.archive.org/web/20120314190054/http://www.botany.uwc.ac.za/ecotree/trunk/woodanatomy2.htm |archive-date=14 March 2012}}</ref> Many older trees may become hollow but may still stand upright for many years.<ref>{{cite web |url=http://cronodon.com/BioTech/Plant_Bodies_Wood.html |title=Xylem and wood |work=BioTech |publisher=Cronodon Museum |access-date=16 July 2012 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502084356/http://cronodon.com/BioTech/Plant_Bodies_Wood.html |url-status=live }}</ref>

===Buds and growth===
{{stack begin}}
[[File:Illustration Quercus robur0.jpg|thumb|upright|Buds, leaves, flowers and fruit of oak (''[[Quercus robur]]'')]]
[[File:Illustration Abies alba0.jpg|thumb|upright|Buds, leaves and reproductive structures of white fir (''[[Abies alba]]'')]]
[[File:Cycas circinalis(draw).jpg|thumb|upright|Form, leaves and reproductive structures of queen sago (''[[Cycas circinalis]]'')]]
{{stack end}}
{{main|Bud}}

[[File:Magnolia Bud.jpg|thumb|left|Dormant ''[[Magnolia]]'' bud]]
Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to climatic conditions; growth normally ceases when conditions are either too cold or too dry. In readiness for the inactive period, trees form [[bud]]s to protect the [[meristem]], the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales. These are thick, small and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in the [[axil]]s of the leaves ready to produce new side shoots. A few trees, such as the [[eucalyptus]], have "naked buds" with no protective scales and some conifers, such as the [[Chamaecyparis lawsoniana|Lawson's cypress]], have no buds but instead have little pockets of meristem concealed among the scale-like leaves.<ref name=buds>{{cite book |title=The World Encyclopedia of Trees |last=Russell |first=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/18 18–19] |url=https://archive.org/details/isbn_9780754812920/page/18 }}</ref>

When growing conditions improve, such as the arrival of warmer weather and the longer days associated with spring in temperate regions, growth starts again. The expanding shoot pushes its way out, shedding the scales in the process. These leave behind scars on the surface of the twig. The whole year's growth may take place in just a few weeks. The new stem is unlignified at first and may be green and downy. The Arecaceae (palms) have their leaves spirally arranged on an unbranched trunk.<ref name=buds/> In some tree species in temperate climates, a second spurt of growth, a [[Lammas growth]] may occur which is believed to be a strategy to compensate for loss of early foliage to insect predators.<ref>{{cite journal |journal=Journal of Experimental Botany |volume=54 |issue=389 |pages=1797–1799 |date=August 2003 |doi=10.1093/jxb/erg225 |title=August-learning about summer |last1=Battey |first1=N. H. |pmid=12869517|doi-access=free }}</ref>

Primary growth is the elongation of the stems and roots. Secondary growth consists of a progressive thickening and strengthening of the tissues as the outer layer of the epidermis is converted into bark and the cambium layer creates new phloem and xylem cells. The bark is inelastic.<ref>{{cite book |title=Biology |last=Campbell |first=Neil A. |author2=Reece, Jane B. |year=2002 |publisher=Pearson Education |isbn=978-0-201-75054-6 |pages=729–730|edition=6th }}</ref> Eventually the growth of a tree slows down and stops and it gets no taller. If damage occurs the tree may in time become hollow.<ref>{{cite book |title=The World Encyclopedia of Trees |last=Russell |first=Tony |author2=Cutler, Catherine |year=2003 |publisher=Lorenz Books |isbn=978-0-7548-1292-0 |pages=[https://archive.org/details/isbn_9780754812920/page/16 16, 27] |url=https://archive.org/details/isbn_9780754812920/page/16 }}</ref>

===Leaves===
{{main|Leaf}}

Leaves are structures specialised for photosynthesis and are arranged on the tree in such a way as to maximise their exposure to light without shading each other.<ref name=Pessarakli/> They are an important investment by the tree and may be thorny or contain [[phytolith]]s, [[lignin]]s, [[tannin]]s or [[poison]]s to discourage herbivory. Trees have evolved leaves in a wide range of shapes and sizes, in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees. They are adapted for life in environments where resources are low or water is scarce. Frozen ground may limit water availability and conifers are often found in colder places at higher altitudes and higher latitudes than broad leaved trees. In conifers such as fir trees, the branches hang down at an angle to the trunk, enabling them to shed snow. In contrast, broad leaved trees in temperate regions deal with winter weather by shedding their leaves. When the days get shorter and the temperature begins to decrease, the leaves no longer make new [[chlorophyll]] and the red and yellow pigments already present in the blades become apparent.<ref name=Pessarakli>{{cite book |author=Pessarakli, Mohammad |title=Handbook of Photosynthesis |url=https://books.google.com/books?id=0_3XqlcKPpwC&pg=PA725 |year=2005 |publisher=CRC Press |isbn=978-0-8247-5839-4 |pages=717–739 |access-date=28 February 2016 |archive-date=21 May 2016 |archive-url=https://web.archive.org/web/20160521021252/https://books.google.com/books?id=0_3XqlcKPpwC&pg=PA725 |url-status=live }}</ref> Synthesis in the leaf of a [[plant hormone]] called [[auxin]] also ceases. This causes the cells at the junction of the [[Petiole (botany)|petiole]] and the twig to weaken until the joint breaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but most leaves remain intact for some time. Other tropical species and those in arid regions may shed all their leaves annually, such as at the start of the dry season.<ref>{{cite book |url=https://books.google.com/books?id=_16xbB2Py_UC&pg=PA734 |title=Biology: Concepts and Applications |author1=Starr, Cecie |author2=Evers, Christine |author3=Starr, Lisa |publisher=Cengage Learning |date=2010 |page=734 |isbn=978-1-4390-4673-9 |access-date=28 February 2016 |archive-date=19 March 2022 |archive-url=https://web.archive.org/web/20220319090500/https://books.google.com/books?id=_16xbB2Py_UC&pg=PA734 |url-status=live }}</ref> Many deciduous trees flower before the new leaves emerge.<ref>{{cite journal |first=Stephen H. |last=Bullock |author2=Solis-Magallanes, J. Arturo |title=Phenology of canopy trees of a tropical deciduous forest in Mexico |journal=Biotropica |volume=22 |issue=1 |date=March 1990 |pages=22–35 |doi=10.2307/2388716 |jstor=2388716|bibcode=1990Biotr..22...22B }}</ref> A few trees do not have true leaves but instead have structures with similar external appearance such as [[Phylloclade]]s – [[Aerial stem modification|modified stem]] structures<ref>{{Cite book |last=Beentje |first=Henk |year=2010 |title=The Kew Plant Glossary |location=Richmond, Surrey |publisher=[[Royal Botanic Gardens, Kew]] |isbn=978-1-84246-422-9}} p. 87.</ref> – as seen in the genus ''[[Phyllocladus]]''.<ref name="pagephyllocladaceae">Page, Christopher N. (1990). "Phyllocladaceae" pp. 317–319. In: Klaus Kubitzki (general editor); Karl U. Kramer and Peter S. Green (volume editors) ''The Families and Genera of Vascular Plants'' volume I. Springer-Verlag: Berlin; Heidelberg, Germany. {{ISBN|978-0-387-51794-0}}</ref>

===Reproduction===
{{further|Plant reproduction|Pollination|Seed dispersal}}

Trees can be [[Pollination|pollinated]] either by wind or by animals, mostly insects. Many angiosperm trees are insect pollinated. Wind pollination may take advantage of increased wind speeds high above the ground.<ref>{{cite web |url=http://www.treesforlife.org.uk/forest/ecological/pollination.html |title=Pollination |publisher=Trees for Life |access-date=14 November 2017 |archive-date=6 October 2014 |archive-url=https://web.archive.org/web/20141006130745/http://www.treesforlife.org.uk/forest/ecological/pollination.html |url-status=live }}</ref> Trees use a variety of methods of [[seed dispersal]]. Some rely on wind, with winged or plumed seeds. Others rely on animals, for example with edible fruits. Others again eject their seeds (ballistic dispersal), or use gravity so that seeds fall and sometimes roll.<ref name="NathanSeidler2006">{{cite journal |last1=Nathan |first1=Ran |author1-link=Ran Nathan |last2=Seidler |first2=Tristram G. |author-link2=Tristram G. Seidler|last3=Plotkin |first3=Joshua B. |author-link3=Joshua B. Plotkin |title=Seed Dispersal and Spatial Pattern in Tropical Trees |journal=PLOS Biology |volume=4 |issue=11 |year=2006 |pages=e344 |doi=10.1371/journal.pbio.0040344|pmid=17048988 |pmc=1609130 |doi-access=free }}</ref>

===Seeds===
{{main|Seed}}

Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, ''[[Sequoiadendron giganteum]]'', produces one of the smallest tree seeds.<ref name="Walker1997">{{cite book |last=Walker |first=Laurence C. |title=Forests: A Naturalist's Guide to Woodland Trees |url=https://books.google.com/books?id=LNJqY52pdSwC&pg=PA56 |year=1997 |publisher=University of Texas Press |isbn=978-0-292-79112-1 |page=56 |access-date=28 February 2016 |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304153834/https://books.google.com/books?id=LNJqY52pdSwC&pg=PA56 |url-status=live }}</ref> The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to [[Seed dispersal|disperse]] their offspring.

[[File:Samara olmo frassino acero.png|thumb|left|Wind dispersed seed of elm (''[[Ulmus]]''), ash (''[[Fraxinus]]'') and maple (''[[Acer (plant)|Acer]]'')]]

For a tree seedling to grow into an adult tree it needs light. If seeds only fell straight to the ground, competition among the concentrated saplings and the shade of the parent would likely prevent it from flourishing. Many seeds such as [[birch]] are small and have papery wings to aid dispersal by the wind. [[Ash (Fraxinus)|Ash]] trees and [[maple]]s have larger seeds with blade shaped wings which spiral down to the ground when released. The [[Ceiba pentandra|kapok]] tree has cottony threads to catch the breeze.<ref name=Dispersal>{{cite web |url=http://www.vtaide.com/png/seed-dispersion.htm |title=How seeds are dispersed |last1=Meng |first1=Alan |author2=Meng, Hui |work=Interactive Assessment Worksheets |access-date=23 July 2012 |archive-date=5 August 2012 |archive-url=https://web.archive.org/web/20120805115217/http://www.vtaide.com/png/seed-dispersion.htm |url-status=live }}</ref>

The seeds of conifers, the largest group of gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.<ref name="EvertEichhorn2004">{{cite book |last1=Evert |first1=Ray F. |author2=Eichhorn, Susan E. |title=Biology of Plants |url=https://books.google.com/books?id=8tz2aB1-jb4C&pg=PA422 |year=2004 |publisher=Macmillan |isbn=978-0-7167-1007-3 |page=422 |access-date=28 February 2016 |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304154119/https://books.google.com/books?id=8tz2aB1-jb4C&pg=PA422 |url-status=live }}</ref> Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the [[jack pine]], and also enriches the forest floor with wood ash and removes competing vegetation.<ref>{{cite web |url=http://www.zephyrus.co.uk/firedispersal.html |title=Fire |last1=Parkin |first1=Dave |author2=Parkin, Marilyn |work=How do the seeds disperse to form new plants? |publisher=Zephyrus |access-date=23 July 2012 |archive-date=22 June 2012 |archive-url=https://web.archive.org/web/20120622181841/http://www.zephyrus.co.uk/firedispersal.html |url-status=live }}</ref> Similarly, a number of angiosperms including ''[[Acacia cyclops]]'' and ''[[Acacia mangium]]'' have seeds that germinate better after exposure to high temperatures.<ref name="BaskinBaskin2001">{{cite book |author1=Baskin, Carol C. |author2=Baskin, Jerry M. |title=Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination |url=https://books.google.com/books?id=uGJL_Ys6wlQC&pg=PA121 |year=2001 |publisher=Elsevier |isbn=978-0-12-080263-0 |pages=121, 260 |access-date=28 February 2016 |archive-date=22 December 2016 |archive-url=https://web.archive.org/web/20161222054555/https://books.google.com/books?id=uGJL_Ys6wlQC&pg=PA121 |url-status=live }}</ref>

The [[Delonix regia|flame tree]] ''Delonix regia'' does not rely on fire but shoots its seeds through the air when the two sides of its long pods crack apart explosively on drying.<ref name=Dispersal/> The miniature cone-like catkins of [[alder]] trees produce seeds that contain small droplets of oil that help disperse the seeds on the surface of water. Mangroves often grow in water and some species have [[propagule]]s, which are buoyant fruits with seeds that start germinating before becoming detached from the parent tree.<ref name="BarbourBillings1999">{{cite book |author1=Barbour, Michael G. |author2=Billings, William Dwight |title=North American Terrestrial Vegetation |url=https://books.google.com/books?id=Ra2QzAh9xdIC&pg=PA528 |year=1999 |publisher=Cambridge University Press |isbn=978-0-521-55986-7 |page=528 |access-date=28 February 2016 |archive-date=22 December 2016 |archive-url=https://web.archive.org/web/20161222052706/https://books.google.com/books?id=Ra2QzAh9xdIC&pg=PA528 |url-status=live }}</ref><ref>{{cite web |url=http://www.naturefoundationsxm.org/education/mangroves/the_white_mangrove.htm |title=The White Mangrove |publisher=Nature Foundation, St. Maarten |author=van der Neut, Marcus |work=naturefoundationsxm.org |url-status=dead |archive-url=https://web.archive.org/web/20120125214433/http://www.naturefoundationsxm.org/education/mangroves/the_white_mangrove.htm |archive-date=25 January 2012 }}</ref> These float on the water and may become lodged on emerging mudbanks and successfully take root.<ref name=Dispersal/>

[[File:Gebarsten bolster van een paardenkastanje (Aesculus) 20-09-2020 (d.j.b.) 01.jpg|thumb|Cracked [[Thorns, spines, and prickles|thorny]] skin of a [[Aesculus tree]] seed|upright]]
Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like [[Crataegus|hawthorns]] have seeds enclosed in edible tissue; animals including mammals and birds eat the fruits and either discard the seeds, or swallow them so they pass through the gut to be deposited in the animal's droppings well away from the parent tree. The germination of some seeds is improved when they are processed in this way.<ref>{{cite web |url=http://www.webpages.uidaho.edu/range556/appl_behave/projects/seed_dispersal.html |title=Seed Dispersal by Animals: Behavior Matters |author=Yang, Suann |work=BEHAVE: Behavioral Education for Human, Animal, Vegetation and Ecosystem Management |access-date=23 July 2012 |archive-date=9 December 2013 |archive-url=https://web.archive.org/web/20131209043548/http://www.webpages.uidaho.edu/range556/appl_behave/projects/seed_dispersal.html |url-status=live }}</ref> [[Nut (fruit)|Nuts]] may be gathered by animals such as squirrels that [[hoarding (animal behavior)|cache]] any not immediately consumed.<ref name="LeveySilva2002">{{cite book |author1=Levey, Douglas J. |author2=Silva, Wesley R. |author3=Galetti, Mauro |title=Seed Dispersal and Frugivory: Ecology, Evolution and Conservation |url=https://books.google.com/books?id=sU7213gPmDMC&pg=PA206 |year=2002 |publisher=CABI |isbn=978-0-85199-525-0 |page=206 |access-date=28 February 2016 |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304160055/https://books.google.com/books?id=sU7213gPmDMC&pg=PA206 |url-status=live }}</ref> Many of these caches are never revisited, the nut-casing softens with rain and frost, and the seed germinates in the spring.<ref>{{cite journal |author1=Ruxton, Graeme D. |author1-link=Graeme Ruxton |author2=Schaefer, H. Martin |year=2012 |title=The conservation physiology of seed dispersal | journal=Philosophical Transactions of the Royal Society | volume=367 | issue=1596 | pages=1708–1718 | doi=10.1098/rstb.2012.0001 |pmid=22566677 |pmc=3350653 |doi-access=free}}</ref> Pine cones may similarly be hoarded by [[red squirrel]]s, and [[grizzly bear]]s may help to disperse the seed by raiding squirrel caches.<ref>{{cite web |url=http://www.webpages.uidaho.edu/range556/Appl_BEHAVE/projects/whitebark_pine.html |title=Whitebark Pine Seeds, Red Squirrels, and Grizzly Bears: An Interconnected Relationship |author=Sager, Kim |work=BEHAVE: Behavioral Education for Human, Animal, Vegetation and Ecosystem Management |access-date=23 July 2012 |archive-date=9 December 2013 |archive-url=https://web.archive.org/web/20131209043552/http://www.webpages.uidaho.edu/range556/Appl_BEHAVE/projects/whitebark_pine.html |url-status=live }}</ref>

The single extant species of Ginkgophyta (''Ginkgo biloba'') has fleshy seeds produced at the ends of short branches on female trees,<ref>{{cite web |url=http://faculty.unlv.edu/landau/gymnosperms.htm |title=Gymnosperms |work=unlv.edu |access-date=27 September 2012 |archive-date=9 October 2012 |archive-url=https://web.archive.org/web/20121009024559/http://faculty.unlv.edu/landau/gymnosperms.htm |url-status=live }}</ref> and ''[[Gnetum]]'', a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis.<ref name="BhatnagarMoitra1996">{{cite book |author1=Bhatnagar, S. P. |author2=Moitra, Alok |title=Gymnosperms |url=https://books.google.com/books?id=4dvyNckni8wC&pg=PA371 |year=1996 |publisher=New Age International |isbn=978-81-224-0792-1 |page=371 |access-date=28 February 2016 |archive-date=4 March 2017 |archive-url=https://web.archive.org/web/20170304152805/https://books.google.com/books?id=4dvyNckni8wC&pg=PA371 |url-status=live }}</ref>